Summary of Work: Rates of spontaneous mutation vary greatly among organisms. We have previously described genomic mutation rates that are characteristic of certain broad groups of organisms. Riboviruses have genomic mutation rates of 0.1?1 per replication, a value approaching the maximum compatible with survival. The first mutational spectrum for a ribovirus (tobacco mosaic virus) displayed a high proportion of mutants carrying multiple mutations, far in excess of predictions based on the mutant frequency and randomly distributed mutations. The characteristic mutation rate for DNA-based microbes is about 0.003 per genome replication. In the Archeon Sulfolobus acidocaldarius growing at 75 degrees C, the mutation rate is close to 0.003 but the proportion of base-pair substitutions is lower than in all other DNA-based organisms. We hypothesized that the average missense mutation has a greater deleterious effect at high temperatures. As a result, evolutionary pressures would act to decrease base-substitution mutation rates. One prediction of this hypothesis is that the ratio of missense to silent (synonymous) mutations accumulating in the course of evolution would be lower in thermophiles than in mesophiles. Another prediction is that a thermophilic eubacterium would also show reduced frequencies of base-pair substitutions but that a mesophilic archaeon would not.